Apparatuses for delivering hot thermoplastic adhesives, sometimes referred to as “hot melt adhesives”, are known in the art. As shown in FIGS. 1 and 2, such apparatuses generally include a heater 12 to continue heating an adhesive and a valve assembly 14 directly attached to the heater 12. The heater 12 generally includes a housing 54 and a heater exit aperture 52 located in the housing for delivering the adhesive from the heater housing 54 to the valve assembly 14. The heater 12 may further include a clamp 49 for attaching the heater 12 to machinery. The heater 12 is connected to an external power source via a power cable 48. The valve assembly 14 generally includes a valve assembly housing 15, a valve assembly adhesive entrance aperture 24 for allowing the adhesive to enter the valve assembly housing 15, a valve assembly adhesive exit aperture or nozzle 20 for allowing the adhesive to exit the valve assembly housing 15, and a valve 17 having an open position in which the valve 17 permits the adhesive to exit the valve assembly housing 15 via the valve assembly adhesive exit aperture 20 and a closed position in which the valve 17 prevents the adhesive from exiting the valve assembly housing 15 via the valve assembly adhesive exit aperture 20. Adhesives may be heated externally then conveyed into the heater housing 54, where the heater 12 continues heating the adhesives. The adhesives then exit the housing 54 of the heater 12 through the heater adhesive exit aperture 52, enter the housing 15 of the valve assembly 14 through the valve assembly adhesive entrance aperture 24, and exit the valve assembly housing 15 through the valve assembly adhesive exit aperture 20. After the exiting the valve assembly adhesive exit aperture 20, the adhesive is ultimately applied to the target surface a number of different ways, including bead dispensing, non-contact spraying, and contact coating.
In some prior art adhesive systems, the valve 17 is opened and closed via electricity, as shown in FIG. 2. In other prior art adhesive systems, the valve 17 is opened via pressurized air, in which case the heater 12 delivers pressurized air to the valve assembly 14. In particular, air exits the housing 54 of the heater 12 through one or more heater to valve assembly air apertures 51, enters the valve assembly housing 15 through one or more valve assembly air apertures 23, and may exit the valve assembly housing 15 through a valve assembly air vent aperture 22, as shown in FIG. 1. In one such design, the system includes two heater to valve assembly air apertures 51 and two valve assembly air apertures 23, as shown in FIG. 1, in which case the valve 17 is both opened and closed via pressurized air. An opened air line may be used to open the valve 17 and a closed air line may be used to close the valve 17. The opened and closed air lines may be two way air lines with air pressure moving the valve stem or piston to the open position and the closed air line venting the air from the closed side of the piston, and the closed air line may be pressurized moving the valve stem or piston to the closed position with the opened air line venting the air from the opened side of the piston. In another design (not shown), the valve assembly 12 includes only one heater to valve assembly air aperture 51 and only one valve assembly air aperture 23, in which case the valve 17 may be opened via pressurized air and closed via a spring. A set screw 22A may be available to set the valve stem or piston travel length.
A problem with the systems described above is that particulate matter in the adhesives can clog the valve assembly adhesive exit aperture or nozzle 20 and/or valve 17 over time, leading to the inoperability of the valve assembly adhesive exit aperture or nozzle 20 and/or valve 17. This problem is significant, given that many valves, such as those produced by Nordson Corp., are located in hard to reach places in the valve assembly housing 15. Ultimately, many users decide to replace the entire valve assembly at a cost of approximately $100 to $1,000.
As illustrated in FIGS. 1 and 2, and described in, for example, U.S. Pat. Nos. 6,799,702 and 6,315,168, an internal filter 68 disposed within the valve assembly housing 15 has been previously described. However, by disposing the filter 68 within the valve assembly housing 15, the filter 68 cannot be replaced without disassembling the valve assembly housing 15.
U.S. Patent Publication No. 2008/0217360 discloses a filter screen that is located in a recess in the housing of the valve assembly so that an adhesive is delivered from the heater adhesive exit aperture 52 through the filter screen and into the valve assembly adhesive entrance aperture 24. However, a potential problem with the system of U.S. Patent Publication No. 2008/0217360 is that contaminants stop the flow of adhesive when the limited area of the valve assembly adhesive entrance aperture 24 becomes covered and clogged as the adhesive flows from the heater adhesive exit aperture 52 through the small filter screen and into the small valve assembly adhesive entrance aperture 24, which could occur in a short period of time.
Therefore, there is a continuing need for new and better apparatuses for filtering hot adhesives.